Conventionally, a detection device has been known which projects pulsed light and detects light reflected from an object, so as to detect presence or absence of the object. Such a detection device is, for example, used in an automatic door, an automatic washing apparatus for a sanitary fixture, or amusement equipment. Such a detection device is, for example, disclosed in Patent Literature 1 (Japanese Unexamined Utility Model Application Publication, Jitsukai, No. 6-18983 (Publication Date: Mar. 14, 1994)) and Patent Literature 2 (Japanese Patent Application Publication, Tokukai, No. 2006-145483 A (Publication Date: Jun. 8, 2006)).
Meanwhile, a new type of portable device has appeared in which a sensor (hereinafter referred to as “proximity sensor”) detecting presence or absence of an object in proximity is provided in a portable device, such as a portable phone or a media player, which has progressed in multifunction and in reduction in size or reduction in thickness. Such a new type of portable device is used for the following purposes.
(1) On/off control of a backlight for a display screen of a portable device having a telephone function and the display screen. The backlight for the display screen is turned off, for example, when a proximity state in which human skin comes close to the portable device is detected during a call. The backlight is turned on again when a change from the proximity state to a non-proximity state is detected. This allows a reduction in power consumption of an entire system.
(2) On/off control of a touch panel function of a portable device having telephone function and touch panel function. The touch panel function is turned off, for example, during a call or while the device is put into a pocket. This allows prevention of malfunction of a system.
(3) A touchless switch of a portable device which has a wireless communication function. For example, a device such as a wireless mouse, a wireless keyboard, or a controller of a game machine is turned on while an operator put his hand or finger close to the device, and the device is put into a sleep mode while the operator put his hand or finger away from the device. This allows a reduction in power consumption of the device.
It is strongly desired that a proximity sensor, used as the applications of the portable device like above, (i) has higher resistance to external disturbance light such as sunlight and fluorescent light than conventional optical sensor technologies, (ii) realizes, with an extremely small mounting area, sensing properties such as desired detection sensitivity and response time, and (iii) has extremely low power consumption.
In order to reduce size of such a detection device without sacrificing detection sensitivity, it is necessary (i) to reduce size and improve sensitivity of a light-receiving section and a light-receiving element and (ii) to realize high sensitivity of an amplifier circuit for a light-reception signal or to realize an increase in light amount of a light-emitting element. However, this causes an increase in power consumption of the amplifier circuit or a light-emitting element driving circuit. In addition, this causes an increase in optical or electrical crosstalk between the light receiving section and a light-emitting section and/or a reduction in resistance to external disturbance light, thereby resulting in that a malfunction is likely to occur.
Moreover, in a case where any of the detection methods disclosed in the respective Patent Literatures is applied to such a detection device having a small size and low power consumption, the following problems are caused: (i) it is difficult to realize a compact modular proximity sensor or a compact monolithic proximity sensor, (ii) a problematic response occurs which is not suitable for a sensor, and (iii) a situation often occurs which is incompatible with or conflicts with the techniques required for suppressing noise and offset so that analog characteristics such as desired detection sensitivity are realized as an actual sensor circuit.
For example, the Patent Literature 1 discloses an object detection device which, in a case where it receives the light during a non-light-emission period of a light-emitting section, suspends its process, until it ceases to receive light, so as to improve the resistance to external disturbance light. FIG. 10 shows a flowchart of this process. Specifically, a light-receiving circuit is activated once every 0.7 seconds, and the object detection device waits, for 5 ms after a start of operation, for external disturbance light to disappear. Thereafter, the object detection device causes the light-emitting section to emit light, and observes presence or absence of corresponding reflection light. If the external disturbance light exists more than 5 ms, it is assumed that it is not possible to observe any object and that no reflecting object is present, and the object detection device waits for the next activation of the light-receiving circuit.
This algorithm causes the following problems.
First, influence of noise contributing factors (e.g. temperature drift of a circuit, and power supply noise) other than the external disturbance light is enormous since external disturbance light is observed during an entire period no light is emitted. Moreover, sampling of a state is carried out at long intervals of 0.7 seconds. Accordingly, in a case where a time period the external disturbance light is sufficiently attenuated is short, or in a case where intensity of the external disturbance light fluctuates so as to have some cycle, then reflection light can be observed only at the timing which is accidentally coincident with the beat cycle. Consequently, the object detection device has a low probability of carrying out object detection operation. Therefore, the object detection device of the Patent Literature 1 cannot meet response and accuracy required for functioning as a proximity sensor such as that tracks successive human movements, which is an object to be detected by the present invention. Moreover, the reflection light is observed to the extent that the number or level of the received pulses with respect to a light-emission pulse pattern is merely counted or sensed, respectively. As such, it is not possible to obtain judgment information required for carrying out a more precise time-axis control with respect to the operation of the detection device. Thus, the object detection device of the Patent Literature 1 has no element which allows precise searching of the timing so that the reflection light can be surely observed under the environment where external disturbance light exists, and therefore has extremely low observation accuracy in view of the time-axis.
Next, according to the technique disclosed in the Patent Literature 2, a pattern of light-emission pulse is designed so that the resistance to external disturbance light is improved (see FIG. 11). Specifically, light emission is repeated more than once at a cycle which is twice as large as light emission pulse width, and a main frequency component of signal light is set to be higher than a frequency component of the external disturbance light. In FIG. 11, the light emission is repeated twice. In view of its purpose, however, this gives rise to a necessity of extending, more than necessary, the bandwidth of transmitting circuit and receiving circuit toward the high frequency band. This causes a detection circuit to detect even noise components of high frequencies, thereby making it difficult to attain high sensitivity. In an algorithm such as that is shown in FIG. 11, a detected output is updated too often, so that chattering of the detected output is caused. Therefore, the technique disclosed in the Patent Literature 2 cannot have sufficient response characteristics as a proximity sensor for tracking successive human movements, which is an object to be detected by the present invention.
Moreover, in a case where high integration, in which analog and digital elements are mixed, is intended by use of a fine CMOS process so as to realize a compact detection device, an offset voltage generated in the integrated circuit due to variations in the elements of the integrated circuit becomes a principal factor which causes a large deterioration in sensitivity of the detection device.